Method for assembling a structural panel in order to prevent the sag thereof

ABSTRACT

A method for facilitating the assembly of a structural panel and for preventing the sag thereof comprises the step of placing the structural panel (2), comprising an outer plate (6) and an underlying skeleton framework (7), upon longitudinally extending support beams (B&#39;) which interconnect the upper ends of laterally spaced rows of longitudinally spaced support columns (C) which extend upwardly from a floor support surface (F). A mobile radiation heating element truck (4), movable upon laterally spaced rails (8) by means of casters (11), includes radiation heating elements (12). The truck (4) is movable between operative successive operative positions, stages, or stations (A,B), and when the truck (4) is disposed at the first station (A), the plate (6), or a section thereof, is preheated to a predetermined temperature so as to thermally expand the same whereupon the truck (4) is then moved to the second station (B) so as to permit the thermally expanded plate (6) disposed at the first station (A) to be tack-welded and finish-welded to the underlying skeleton framework (7 ) while a second plate (6), or section thereof, is preheated to a predetermined temperature at the second station (B). Upon cooling of the heated plate (6) disposed at the first station (A), the plate (6) develops a residual tensile stress due to thermal shrinkage deformation whereby sag of the panel is effectively prevented. The support of the panel (2), including the plate (6) and the skeleton framework (7) at the elevated level or height above floor (F) provides substantially unobstructed access to the undersurface regions of the plate (6) and the skeleton framework (7) so as to facilitate the welding operations. Movement of the truck (4) to the next operative position or station (B) likwise provides substantially unobstructed access to the upper surfaces of the plate (6) and skeleton framework (7) so as to likewise facilitate the welding operations to be performed within such regions.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 246,933 filed on Sept. 19, 1988, abandoned whichis, in turn, a divisional patent application of U.S. patent applicationSer. No. 930,185 filed Nov. 13, 1986, abandoned.

FIELD OF THE INVENTION

The present invention relates generally to a method for forming orassembling a structural panel, and more particularly to a method forpreventing the occurrence of "sag" during the formation or assembly of astructural panel which is to be used in the fabrication or assembly ofroof structures, side wall structures of vehicles, ships, buildings, andthe like.

BACKGROUND OF THE INVENTION

In recent years, lightening of the aforenoted types of structures hasbeen especially desirable, and a reduction in the thickness dimensionsof the structures has accordingly been promoted in an effort to achievethis objective. Soft steel, stainless steel, aluminum alloys, and thelike, are usually applied as outer plates for such panels. With theaforenoted promotion of the reduction in the thickness dimensions of thepanel structures, however, the occurrence of "sag" within the outerplate member, after completion of the assemblage and welding operationsof the outer plates to the underlying structural or support beams,inevitably arises, and problems are therefore posed with respect to thelocal strength of the "sagging" part of the panel as well as theexternal appearance of the panel.

With the intention of eliminating the aforenoted distortion effects ofthe welding process, there has accordingly been disclosed a techniquewherein a large number of sheets of standard shape are formed into anelongated outer plate by means of a welding operation, whereupon underthe state in which the outer plate is loaded or subjected to a stressedor tensioned state, such plate structures are then attached to askeleton or structural framework, as disclosed, for example, within theofficial gazaette of Japanese Patent Application Publication No.53-39261; or a technique wherein the aforenoted elongated outer platesis loaded in a tensioned state or with tensile stress and is alsopreheated so as to thermally expand the same, whereupon the outer platecan be attached to the skeleton or structural framework, as shown, forexample, within the official gazette of Japanese Patent ApplicationPublication No. 54-20185. Both of these techniques, however, require alarge-scale tensioning and heating system which involve enormousinstallation costs as well as a large number of process steps.

Still further, another drawback of such system is that it is verydifficult to precisely control the system so as to achieve apredetermined heating temperature.

Yet still further, there has also been provided a technique wherein anouter plate of a comparatively small size is disposed within a heatingbox or oven, and the entire outer plate is heated to a predeterminedtemperature so as to be thereafter welded to a skeleton or structuralframework as shown for example within the official gazette of JapanesePatent Application Laid-Open No. 60-64791. In accordance with thistechnique, however, the heating box or oven is quite complex instructure, and moreover, as the larger plate becomes larger in size, theheating box or over necessarily needs to be proportionally larger insize so as to accommodate the larger plate and panel structure, wherebyproblems such as, for example, a large floor space and a workshop oflarge area are incurred, and in addition, the installation costs becomesignificantly high.

OBJECT OF THE INVENTION

In view of the aforenoted problems characteristic of the prior art, thepresent invention has for its primary objective the provision of amethod for preventing the sag of a structural panel wherein theoccurrence of "sag" during the assembly of the outer plate members uponthe underlying skeleton or structural beam or framework portion of thestructural panel is effectively prevented by employing unique processingtechniques and operations.

SUMMARY OF THE INVENTION

In accordance with the aforenoted objective of the present invention,the method for accomplishing the assembly of the structural panel bymeans of the processing techniques and operations uniquelycharacteristic of the present invention comprises laterally spacing apair of rows of support columns which project vertically upwardly from afloor support surface such that the columns are also longitudinallyspaced with respect to each other within each one of the laterallyspaced rows, and mounting longitudinally extending beams upon the upperends of the columns so as to interconnect the same. A skeletonframework, comprising laterally and longitudinally extending panelstructural beams interconnected together, is then mounted upon thelongitudinally extending beams disposed atop the support columns, and anouter plate of the composite structural panel is then disposed upon thethe skeleton framework. Radiation heating means is subsequently disposedabove the upper surface of the outer plate of the composite structuralpanel so as to preheat the outer plate to a predetermined temperature inorder to thermally expand the same prior to and in preparation for thewelding of the outer plate to the upper surface of the underlyingskeleton framework under such thermally expanded conditions. In thismanner, residual tensile stresses are developed within the plate andstructural panel as a result of thermal shrinkage deformation of theouter plate during the cooling time period subsequent to completion ofthe welding operation whereby the occurrence of "sag" within the plateand structural panel is effectively prevented.

The radiation heating means comprises a mobile truck which is movablymounted upon a pair of laterally spaced rails disposed laterallyoutwardly of the vertically upwardly projecting support columns. Thetruck includes downwardly facing radiation heating elements supportedupon the undersurface of a radiation heating element platform or panel,and the radiation heating element platform or panel is supported bymeans of downwardly depending legs, disposed at each corner of theplatform or panel, the lower ends of the legs being provided with casterwheels or the like for riding upon the laterally spaced rails. In thismanner, the mobile truck, with the radiation heating means disposedthereon, may be movably disposed along the rails such that the radiationheating platform or panel, with the radiation heating elementincorporated therein, is disposed above the composite structural panelfor accomplishing the aforenoted preheating operation with respect tothe outer plate and may be subsequently moved away from the compositestructural panel so as to achieve the aforenoted welding operation. Whenthe mobile truck is moved away from the preheated composite structuralpanel to a location remote from the structural panel, the heating meansmay be utilized to preheat a successive structural panel, or asuccessive section of the same structural panel, supported upon thevertical support columns and longitudinally connecting beams, such thatfabrication of multiple panels or multiple panel sections may beaccomplished in a mass-production mode.

It is to be noted further that the elevation of the structural panel,and in particular, the outer plate thereof, at a substantial heightabove the supporting floor surface, as a result of the provision of thevertically upstanding support columns, provides substantiallyunobstructed access for welding personnel and/or equipment to theundersurface portions of the outer plate, and the junctions definedbetween the outer plate and the upper surfaces of the skeleton frameworkbeams, in order for such outer plate and skeleton beam framework to besecured together by means of, for example, spot welding techniques.Movement of the mobile truck, carrying the radiation heating means, toits remote location following the outer plate preheating thermaltreatment likewise provides substantially unobstructed access for suchwelding personnel and machinery or equipment to the upper surfaceregions of the outer plate, as well as the junctions defined between theouter plate and the upper surfaces of the skeleton framework, whereby,for example, spot welding techniques can also be employed with respectto the upper surface regions of the composite structural panel.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and attendant advantages of the presentinvention will become better understood from the following detaileddescription of the invention, when considered in conjunction with theaccompanying drawings, in which like reference characters designate likeor corresponding parts throughout the several views, and wherein:

FIG. 1 is a perspective view of the composite structural panelfabricating apparatus utilized for assembling the composite structuralpanels in accordance with the method and techniques of the presentinvention;

FIG. 1A is a plan view of a structural panel fabricated by means of themethod and techniques of the present invention;

FIG. 2 is a front view, partly in section, of the radiation heatingmeans utilized in accordance with the method and techniques of thepresent invention;

FIG. 3 is a bottom plan view of the radiation heating means shown inFIG. 2;

FIG. 4 is a graphical diagram illustrating the cooling characteristicsof a plate during the natural cooling stage following the heatingthereof; and

FIGS. 5-7 are diagrams showing the temperature distributions within atest piece plate during a heating test thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, the various system components, utilizedfor carrying out the method and techniques of the present invention,will be described. In particular, the system for carrying out the panelfabricating method and techniques of the present invention is generallydesignated by the reference character 1 as shown in FIG. 1, and thesystem is exemplary of that type of system which may be used tofabricate a composite structural panel 2 for railway vehicles. Thevarious components of the fabrication system for achieving the methodand techniques in accordance with the present invention is adapted to besupported upon a support surface or floor F, and is seen to generallycomprise support means 3, a radiation heating element truck 4, andradiation heating element means 5 supported by means of truck 4.

More particularly, the support means 3 comprises laterally spaced rowsof upstanding support columns C, with the support columns C disposedwithin each row also being longitudinally spaced from each other.Longitudinally extending beams B' are disposed atop the upper ends ofthe upstanding support columns C so as to interconnect the same, and astructural panel skeleton framework, comprising longitudinally andlaterally extending skeleton beams 7, as more particularly shown in FIG.1A, has its opposite side edges resting upon the longitudinallyextending support beams B' so as to be able to support a rectangularouter plate 6 upon the upper surface thereof whereby the outer plate 6and skeleton framework 7 comprise the composite structural panel to befabricated in accordance with the method and processing techniques ofthe present invention.

A pair of longitudinally extending, laterally spaced rails 8 are fixedlysecured to floor surface F at positions disposed laterally outwardly ofthe upstanding support columns C, and the radiation heating elementtruck 4 is movably disposed upon the rails 8 between various processingstations A, B, and the like, as will be more apparent hereinafter. Theheating element truck 4 is seen to comprise a rectangular framework 9,dependent support legs 10 extending downwardly from the corners of theframework 9, and caster members 11 disposed upon the lower ends of thelegs 10 so as to be capable of rolling upon the rails 8. In this manner,the framework 9 of the heating element truck is disposed above the uppersurface of the outer plate 6, which is disposed upon the skeletonframework 7, in order to support the heating elements per se at aposition above the upper surface of the outer plate 6. In particular,the radiation heating means 5 is supported within the framework 9, andas best seen in FIG. 3, heating means 5 comprises a plurality of heatingelement support panels or units 13 each of which has supported thereon aplurality of downwardly extending and downwardly radiating heatingelements 12 which may comprise, for example, infrared lamps.

With respect to the particular structure of the heating element mountingframework 9, it is seen from FIGS. 2 and 3 that the framework 9comprises a marginal frame member 9a, longitudinally extending framemembers or beams 9b, and transversely extending crossbeams 9c, wherebythe radiation heating means 5 is divided by means of the frameworkmembers 9a-9c into a plurality of regions or units of identical shape.Each heating unit 13 includes twelve heating elements or lamps 12, and apower or feeder cord 15, as seen in FIG. 1, which has a plurality ofsections thereof suspended from hook members 17 operatively mounted in aslidable manner from the underside of a guide rail 14 so as to permitfree extension or contraction of the power cord 15 in conjunction withthe movement of the mobile truck 4 and the heating elements or units 13carried thereby, is electrically connected to the respective heatingunits 13 so as to provide electrical power thereto. According to theparticular material comprising the outer plate 6 to be heat-treated, aswell as the thickness and surface area thereof, the number of heatingunits 13 to be installed or accommodated upon the mobile truck 4, and tobe provided with electrical power, or to have a particular level ofelectrical power supplied thereto, can of course be varied so as to beaccordingly increased or decreased. Similarly, the number of heatingelements 12 to be disposed upon each heating unit 13, or the heatingcapacity or power of each element 12, can likewise be varied byincreasing or decreasing the same. Still further, while it has beennoted that heating elements 12 may comprise infrared lamps, infraredheating elements of a type other than infrared lamps may of course beemployed.

In order to control the heating temperatures or temperature levels towhich the outer plates 6 and composite structural panels are to besubjected, temperature sensors 16, as shown in FIG. 1, are disposed atpredetermined locations upon the upper surfaces of the outer plates 6.It has been experimentally revealed that for a panel of the size shownin FIG. 5, one or two sensors 16 will suffice for properly controllingthe radiation heating means 5.

Continuing further, the fabrication operation for assembling the outerplate 6 to the underlying skeletal framework structure 7, in accordancewith the processing method and techniques of the present invention, soas to prevent the occurrence of sag of the panel 2, will now bedescribed. Initially, the skeletal framework 7 is placed upon the upperends of the upstanding columns 3, and in particular, upon thelongitudinally extending support beams B'. The outer plate 6 is thendisposed upon the the skeletal framework 7, and as can be readilyappreciated from FIGS. 1 and 1A, the outer plate 6 in fact comprises aplurality of outer plate sections 6. In accordance with the method andprocessing techniques of the present invention, once an outer platesection 6 is placed upon the skeletal framework 7, the radiation heatingelement truck 4 is then moved to a first operative position or stationdesignated as stage A as seen in FIG. 1 so as to heat the first orforward section of the entire outer plate 6 by means of the radiationheating element means 5 being disposed over or above the first orforward section of the outer plate 6 disposed at stage or station A. Theheating elements 12 are then appropriately energized so as to in factpreheat the outer plate 6 disposed at stage or station A to apredetermined temperature T, as will be more fully referred tohereinafter, so as to thermally expand the outer plate section 6. Atthis time, it is to be additionally appreciated that not only is theouter plate section 6 heated by means of radiant heat from heatingelements 12 of the heating means 5, but in addition, the skeletonframework 7 is heated by means of conduction from the preheated outerplate section 6. However, since the surface area portions of theskeleton framework which are in contact with the outer plate section 6are relatively small, the heat conduction within the skeleton framework7 is relatively insignificant.

When the plate section 6 disposed at station or stage A reaches itspredetermined temperature level, the heating element truck 4 is movedaway from its first operative position at stage or station A so as tosubsequently or successively dispose its heat radiation means 5 at stageor station B at which a second outer plate section 6 may be preheated toits desired preheated temperature level for achieving thermal expansionthereof. While the heating element truck 4 is therefore disposed at itssecond operative position at stage or station B so as to preheat thesecond outer plate section 6 disposed at station or stage B, the firstouter plate section 6 disposed at stage or station A is quickly tackedand then finish-welded to the skeleton framework 7. The operations mayof course be repeated successively such that succeeding outer platesections 6 downstream of stage B may be similarly heated, thermallyexpanded, tacked, and finish-welded. In connection with the weldingoperations, it has been experimentally verified that if the tack weldingis promptly and reliably performed, the plate section 6 within whichheat still remains will not develop any considerable tensile stresses,however, the plate 6 will in fact generate sufficient residual tensilestresses after completion of the finish-welding operations. Moreparticularly, upon completion of the fixation of the outer plate section6 to the skeletal framework 7 by means of, for example, spot welding orfinish-welding techniques, the plate 6 in fact develops the properresidual tensile stresses therein due to thermal shrinkage deformationattendant the subsequent natural cooling thereof whereby the completedfabrication of the structural panel 2 comprising the outer plate 6 andskeleton framework 7 is achieved without the occurrence of any "sag". Itis further noted at this time that in connection with the performance ofthe finish-welding operations, spot welding techniques are preferred inconnection with the structural panels of this type with which thepresent invention is operatively associated, however, continuous weldingtechniques, such as, for example, arc welding, are not desirable in viewof the fact that such latter techniques produce undesirable distortionswithin the panels.

With respect to the control of the degree or level to which the outerplate sections 6 are heated, temperature level control is achieved bymeans of the aforenoted thermal or temperature sensors 16. Electricalsignals generated by means of the temperature sensors 16 are thenappropriately utilized to control the energization circuits of theheating elements 12. In particular, and as one example of thetemperature level control, one of the sensors 16 may be set at apredetermined low level temperature while another one of the sensors 16may be set at a predetermined high level temperature so as to in effectcontrol the heating temperature to a value therebetween.

It is to be further noted at this juncture that the fabrication of thestructural panel 2 comprising the outer plate sections 6 and theunderlying skeleton framework 7 is particularly facilitated by means ofthe present invention method and techniques in order to in fact achievea panel free from "sag" and any residual distortion because, as a resultof the provision of the support means 3, comprising the verticallyupstanding columns C and the longitudinally extending support beams B'disposed atop the upper ends of the support columns C, the structuralpanel 2 is disposed at a position which is elevated a significantdistance above the floor support surface F. In this manner, the spotwelding operations can in fact be performed in view of the fact thatsuch elevated disposition of the structural panel provides substantiallyunobstructed access to the undersurface of the outer plate 6 and theskeleton framework 7 whereby the spot welding operations can in fact beefficiently performed. Similarly, in view of the mobile heating elementtruck 4, and its movement along the rails from its first operativeposition at stage or station A to its second operative position at stageor station B, substantially unobstructed access to the upper surfaces ofthe outer plate 6 and the skeleton framework 7 is facilitated withrespect to the outer panel section disposed at station A whereby spotwelding operations can likewise be performed from positions above theplane of the structural panel 2. In order to properly achieve thewelding operations and the fabrication of the composite structural panel2, the welding operations are in fact performed from positions bothabove and below the plane of the structural panel 2, and again, theelevated disposition of the panels 2 by means of the upstanding columnsC and the longitudinally extending beams B', plays a critically andintegrally important part in facilitating the assembling operations,processing, and techniques of the present invention. It is additionallynoted that as a result of the aforenoted elevated disposition of thecomposite structural panel 2, large-scale transportation or conveyingmachinery or equipment is enabled to be disposed beneath the panels 2 inpreparation for transportation or conveyance of the same, or forplacement of the skeleton framework members 7 upon the support means 3.Such machinery or equipment may be, for example, fork-lift or similartype apparatus or equipment conventionally utilized within theconstruction industry.

Referring again to the preheating portion of the structural panelfabricating process, that is, the preheating operation performed uponthe outer plate 6, the preheated temperature T to which the outer plate6 is to be heated is determined such that the plate 6 may in factgenerate the required residual tensile stress, and such temperature T isa function of, or must be considered with respect to, the materialcomprising the plate 6, the ambient temperature, the strength of theskeleton framework 7, the procedures for handling the outer plate afterthe pre-heating thereof, the period of time extending from thecompletion of the tack-welding operation to the commencement of thefinish-welding operation, and the like. An example of the predeterminedtemperature T will now be calculated with respect to a case wherein astainless steel plate 1.5 mm thick is employed as the plate 6, theambient temperature is approximately 20° C., and the period of timeoccurring from the time the plate 6 has been preheated and tack-weldeduntil the finish-welding operation is commenced is approximately 1minute.

In accordance with such calculations, it is known that the residualtensile stress necessary for preventing the occurrence of any weldingdistortion of the outer plate 6 upon completion of the welding processesis approximately 15 kg/mm². The temperature difference X, between theouter plate 6 and the skeleton framework 7 upon which the outer plate 6is disposed and is to be fixedly secured by means of the weldingoperations, for developing the aforenoted residual tensile stress, isevaluated by means of the following formula:

    X=k.δ/δy wherein

δ: residual tensile stress;

δy: yield stress or proof stress value;

k: temperature difference at which the tensile stress corresponding toδy develops.

When δ=15 kg/mm², δy=52.7 KG/mm², and k=154° C., are respectivelysubstituted into the above-noted formula, the temperature difference Xis approximately 44° C. Concomitantly, the cooling rate of the stainlesssteel plate, when the surface temperature thereof is approximately 90°C., is determined to be approximately 10° C./minute as is readilydetermined from the graphical diagram of the cooling curvecharacteristic of such stainless steel plate after the heating thereof,as shown, for example, within FIG. 4. The temperature difference X is tobe determined at that point in time at which the welding fixationoperation performed upon the outer plate 6 relative to the underlyingskeleton framework 7 for securing the plate 6 to the framework 7 hasbeen completed. Therefore, assuming that the period of time occurringbetween the completion of the heating operation and the completion ofthe tack welding operation is approximately one minute, and that theambient temperature, that is, the temperature of the skeleton framework7, is 20° C., then the target heating temperature for the outer plate 6is calculated to be 20° C.+44° C.+10° C.=74° C. Accordingly, thepredetermined temperature T of the outer plate 6 during the productionof the composite structural panel 2 should desirably be set atapproximately 80° C. whereby some allowance is made in connection withthe aforenoted or calculated target heating temperature.

In connection with the foregoing, there will now be described theessential points of various heating tests which have been conducted inconnection with the uniform heating to 80° C. of stainless steel platesor various shapes or configurations which are to be utilized as theouter plates 6 of the composite structural panels 2. FIG. 5 shows thesurface temperatures of a plate which were measured when a test piece orplate P₁ comprising a stainless steel plate 1245 mm wide, 900 mm long,and 1.5 mm thick, was subjected to a heating test in which infraredheating lamps, utilized as the heating elements, were energized. Thisheating test was conducted under the additional test conditions whereinthe heating elements comprised thirty-six infrared heating lamps eachhaving a power of 250 W, the distance between the heating elements andthe test piece P₁ was 175 mm, the heating time period was five minutes,and the ambient temperature was 29° C. The results obtained included ahigh temperature of 95° C., a low temperature of 58° C., and an averagetemperature of 80° C. In order to insure a sufficient temperaturedifferential, the plate should preferably be heated for a somewhatlonger period of time.

Continuing still further, FIG. 6 discloses surface temperatures whichwere measured in connection with a test piece P₂ which was made from astainless steel plate 1245 mm wide, 2400 mm long, and 1.5 mm thick andwhich was subjected to a heating test in which infrared heating lampswere again employed. Seventy-two heating elements or heating lamps, eachhaving a power of 250 W, were employed, the distance between the heatingelements and the test piece P₂ was 175 mm, the heating time period wasfive minutes, and the ambient temperature was 22° C. The resultsobtained included a high temperature of 107° C., a low temperature of66° C., and an average temperature of 86° C.

Still yet further, FIG. 7 discloses surface temperatures which weremeasured in connection with a test piece P₃ which was made from astainless steel plate 1150 mm wide, 1500 mm long, and 1.5 mm thick andwhich was subjected to a heating test using infrared heaters as theheating elements. This heating test was conducted under the conditionswherein four heating elements were employed, each of which had a powerof 1 kW, the distance defined between the heating elements and the testpiece P₃ was 20 mm, the heating time period was three minutes, and theambient temperature was 28° C. The results obtained included a hightemperature of 130° C., a low temperature of 73° C., and an averagetemperature of 102° C. Although the average temperature in this caseexceeds the predetermined temperature T=80° C. as noted above, thetemperature can be controlled so as to achieve the aforenotedappropriate temperature level by shortening the heating time period,adjusting the arrangement of the heating elements, or increasing thedistance between the heating elements and the test piece.

It is to be noted that as disclosed within the various embodiments orexamples of the present invention as noted above, only a structuralpanel having a planar configuration has been illustrated as comprisingthe outer plate 6 and the skeleton framework 7, however, if the skeletonframework could exhibit a sufficient degree of strength and couldsatisfactorily endure or achieve the appropriate residual tensilestress, then a structural panel having a curved or arcuate configurationand outer surface could likewise be able to be produced in accordancewith the fabrication process and techniques of the present invention.Accordingly, the radiation heating means 5 should be formed so as tohave a similarly shaped configuration.

The present invention comprises the method and processing techniques asdescribed above, and according to a first aspect of performance of thepresent invention, the occurrence of "sag" within the outer plate of thecomposite structural panel can be effectively prevented withoutrestricting the size of the plate per se, and the number of heatingelements or heat sources can be adjusted at will in conformity with thematerial and dimensions of the plate. In addition, the radiation energyand heat can be readily adjusted and very simply controlled throughmeans of the use of appropriate sensors.

According to a second aspect of the present invention, and theperformance of the process method and techniques thereof, radiationheating means is provided so as to be movable relative to apredetermined position at which the outer plate and skeleton frameworkare disposed so as to initially preheat the outer plate, or a sectionthereof, in order to thermally expand the same, and whereinsubsequently, the radiation heating means is moved to a successiveoperation position, stage, or station at which a second or succeedingouter plate or section thereof, and the associated skeleton framework,are disposed for undergoing a similar preheating operation. At thistime, the preceding plate and framework undergo tack welding andfinish-welding operations at the first predetermined position, stage, orstation, movement of the radiation heating means to the second orsuccessive position, stage, or station facilitating substantially freeand unobstructed access to the preceding structural panel and its plateand skeleton framework for performance of the welding operations. Stillfurther, the radiation heating means can be readily adapted to changesin size and thickness of the outer plate, and the material thereof, byadjusting the number, placement, power, and the like, of the particularheating elements employed. Accordingly, this aspect of the presentinvention brings forth such excellent effects in the fabrication of astructural panel in that the quality of the panel produced is in factenhanced, and conventional sag-eliminating operations normally performedafter completion of the welding operations are able to be dispensed withthereby sharply reducing the number of processing steps of thefabrication operation or process. Still further, the present inventionis particularly well-suited to a flow-type production process wherebyoperations are rendered extremely efficient in a mass-production mode.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the presentinvention can be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be protected by means of LettersPatent of the United States of America, is:
 1. A method for preventingsag due to welding within a structural panel of substantially flatconstruction and comprising a skeleton framework and a thin plate whichis to be secured to said skeleton framework, comprising the stepsof:providing a support surface; laterally spacing a pair of verticallyupstanding support means upon said support surface; placing saidskeleton framework upon upper end portions of said laterally spaced pairof vertically upstanding support means such that opposite sides of saidskeleton framework are supported upon said upper end portions of saidsupport means at an elevated level above said support surface so as toprovide substantially unobstructed access to undersurface portions ofsaid structural panel, formed by said skeleton framework and said plateafter said plate has been placed upon said skeleton framework, as wellas junction regions defined between under-surface portions of said plateand said skeleton framework after said plate has been placed upon saidskeleton framework; providing mobile radiation heating means which ismovable between first and second positions; moving said radiationheating means to said first position at which said radiation heatingmeans, having downwardly projecting radiation heating elements, isdisposed over said plate disposed upon said skeleton framework;energizing said radiation heating means so as to heat said plate to apredetermined temperature level at which said plate achieves thermalexpansion; moving said radiation heating means to said second positionremote from said thin plate when said thin plate has been heated to saidpredetermined temperature level and has achieved said thermal expansionso as to provide substantially unobstructed access to upper surfaceportions of said panel formed by said skeleton framework and said plate,as well as junction regions defined between upper surface portions ofsaid plate and said skeleton framework, in order to facilitatesecurement of said plate to said skeleton framework within the vicinityof said upper and undersurface portions of said structural panel, aswell as within said junction regions defined between said undersurfaceand upper surface portions of said plate and said skeleton framework;welding said plate to said skeleton framework within said junctionregions defined between said upper surface portions and saidundersurface portions of said plate, and said skeleton framework, aspermitted by said unobstructed access to said junction regions definedbetween said upper surface portions and said undersurface portions ofsaid plate, and said skeleton framework, as well as said upper andundersurface portions of said panel formed by said plate and saidskeleton framework; and permitting said thin plate to cool so as toachieve thermal shrinkage deformation and the development ofpredetermined residual tensile stresses therewithin whereby the sag ofsaid panel is effectively prevented.
 2. A method as set forth in claim1, further comprising:controlling said heating of said plate to saidpredetermined temperature level by said radiation heating means by usingat least one temperature sensor.
 3. A method as set forth in claim 1,wherein:said skeleton framework and said plate have a length such thatwhen said radiation heating means is moved to said remote secondposition, said radiation heating means can be used to heat a secondsection of said thin plate while said welding operation is beingperformed upon the originally heated section of said plate at said firstposition.
 4. A method as set forth in claim 2, wherein:said at least onetemperature sensor comprises a single sensor for providing electricalpower to said radiation heating means when the temperature of said plateis below said predetermined temperature and will terminate saidelectrical power to said radiation heating means when said temperaturelevel of said plate reaches said predetermined temperature level.
 5. Amethod as set forth in claim 2, wherein:said at least one temperaturesensor comprises a pair of temperature sensors defining upper and lowertemperature levels so as to maintain the temperature of said platewithin said predetermined temperature levels.
 6. A method as set forthin claim 1, wherein:infrared lamps are used as said radiation heatingmeans.
 7. A method as set forth in claim 1, wherein:infrared heaters areused as said radiation heating means.